/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.transforms;



import com.bff.gaia.unified.sdk.coders.*;

import com.bff.gaia.unified.vendor.guava.com.google.common.base.MoreObjects;

import com.bff.gaia.unified.sdk.coders.AtomicCoder;

import com.bff.gaia.unified.sdk.coders.BigEndianLongCoder;

import com.bff.gaia.unified.sdk.coders.Coder;

import com.bff.gaia.unified.sdk.coders.CoderException;

import com.bff.gaia.unified.sdk.coders.CoderRegistry;

import com.bff.gaia.unified.sdk.coders.DoubleCoder;



import java.io.IOException;

import java.io.InputStream;

import java.io.OutputStream;

import java.util.Objects;



/**

 * {@code PTransform}s for computing the arithmetic mean (a.k.a. average) of the elements in a

 * {@code PCollection}, or the mean of the values associated with each key in a {@code PCollection}

 * of {@code KV}s.

 *

 * <p>Example 1: get the mean of a {@code PCollection} of {@code Long}s.

 *

 * <pre>{@code

 * PCollection<Long> input = ...;

 * PCollection<Double> mean = input.apply(Mean.<Long>globally());

 * }</pre>

 *

 * <p>Example 2: calculate the mean of the {@code Integer}s associated with each unique key (which

 * is of type {@code String}).

 *

 * <pre>{@code

 * PCollection<KV<String, Integer>> input = ...;

 * PCollection<KV<String, Double>> meanPerKey =

 *     input.apply(Mean.<String, Integer>perKey());

 * }</pre>

 */

public class Mean {



  private Mean() {} // Namespace only



  /**

   * Returns a {@code PTransform} that takes an input {@code PCollection<NumT>} and returns a {@code

   * PCollection<Double>} whose contents is the mean of the input {@code PCollection}'s elements, or

   * {@code 0} if there are no elements.

   *

   * @param <NumT> the type of the {@code Number}s being combined

   */

  public static <NumT extends Number> Combine.Globally<NumT, Double> globally() {

    return Combine.globally(Mean.of());

  }



  /**

   * Returns a {@code PTransform} that takes an input {@code PCollection<KV<K, N>>} and returns a

   * {@code PCollection<KV<K, Double>>} that contains an output element mapping each distinct key in

   * the input {@code PCollection} to the mean of the values associated with that key in the input

   * {@code PCollection}.

   *

   * <p>See {@link Combine.PerKey} for how this affects timestamps and bucketing.

   *

   * @param <K> the type of the keys

   * @param <NumT> the type of the {@code Number}s being combined

   */

  public static <K, NumT extends Number> Combine.PerKey<K, NumT, Double> perKey() {

    return Combine.perKey(Mean.of());

  }



  /**

   * A {@code Combine.CombineFn} that computes the arithmetic mean (a.k.a. average) of an {@code

   * Iterable} of numbers of type {@code N}, useful as an argument to {@link Combine#globally} or

   * {@link Combine#perKey}.

   *

   * <p>Returns {@code Double.NaN} if combining zero elements.

   *

   * @param <NumT> the type of the {@code Number}s being combined

   */

  public static <NumT extends Number>

      Combine.AccumulatingCombineFn<NumT, CountSum<NumT>, Double> of() {

    return new MeanFn<>();

  }



  /////////////////////////////////////////////////////////////////////////////



  private static class MeanFn<NumT extends Number>

      extends Combine.AccumulatingCombineFn<NumT, CountSum<NumT>, Double> {

    /**

     * Constructs a combining function that computes the mean over a collection of values of type

     * {@code N}.

     */

    @Override

    public CountSum<NumT> createAccumulator() {

      return new CountSum<>();

    }



    @Override

    public Coder<CountSum<NumT>> getAccumulatorCoder(

		CoderRegistry registry, Coder<NumT> inputCoder) {

      return new CountSumCoder<>();

    }

  }



  /** Accumulator class for {@link MeanFn}. */

  static class CountSum<NumT extends Number> implements Combine.AccumulatingCombineFn.Accumulator<NumT, CountSum<NumT>, Double> {



    long count = 0;

    double sum = 0.0;



    public CountSum() {

      this(0, 0);

    }



    public CountSum(long count, double sum) {

      this.count = count;

      this.sum = sum;

    }



    @Override

    public void addInput(NumT element) {

      count++;

      sum += element.doubleValue();

    }



    @Override

    public void mergeAccumulator(CountSum<NumT> accumulator) {

      count += accumulator.count;

      sum += accumulator.sum;

    }



    @Override

    public Double extractOutput() {

      return count == 0 ? Double.NaN : sum / count;

    }



    @Override

    public boolean equals(Object other) {

      if (!(other instanceof CountSum)) {

        return false;

      }

      @SuppressWarnings("unchecked")

      CountSum<?> otherCountSum = (CountSum<?>) other;

      return (count == otherCountSum.count) && (sum == otherCountSum.sum);

    }



    @Override

    public int hashCode() {

      return Objects.hash(count, sum);

    }



    @Override

    public String toString() {

      return MoreObjects.toStringHelper(this).add("count", count).add("sum", sum).toString();

    }

  }



  static class CountSumCoder<NumT extends Number> extends AtomicCoder<CountSum<NumT>> {

    private static final Coder<Long> LONG_CODER = BigEndianLongCoder.of();

    private static final Coder<Double> DOUBLE_CODER = DoubleCoder.of();



    @Override

    public void encode(CountSum<NumT> value, OutputStream outStream)

        throws CoderException, IOException {

      LONG_CODER.encode(value.count, outStream);

      DOUBLE_CODER.encode(value.sum, outStream);

    }



    @Override

    public CountSum<NumT> decode(InputStream inStream) throws CoderException, IOException {

      return new CountSum<>(LONG_CODER.decode(inStream), DOUBLE_CODER.decode(inStream));

    }



    @Override

    public void verifyDeterministic() throws NonDeterministicException {

      LONG_CODER.verifyDeterministic();

      DOUBLE_CODER.verifyDeterministic();

    }

  }

}